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Page:Popular Science Monthly Volume 57.djvu/523

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513
CHAPTERS ON THE STARS.

apparent movement of the star. Hence our actual result in the case of each separate star is a relation between the motion of the star and the motion of the sun.

I say the motion of the sun and not of the earth, because although the observer is actually on the earth, yet the latter never leaves the neighborhood of the sun, and, as a matter of fact, the ultimate result in the long run must be a motion relative to the sun itself as if we made our observations from that body. The question then arises whether there is any criterion for determining how much of the apparent motion of any given star should be attributed to the star itself and how much to a motion of the sun in the opposite direction.

If we should find that the stars, in consequence of their proper motions, all appeared to move in the same direction, we would naturally assume that they were at rest and the sun in motion. A conclusion of this sort was first reached by Herschel, who observed that among the stars having notable proper motions there was a general tendency to move from the direction of the constellation Hercules, which is in the northern hemisphere, towards the opposite constellation Argo, in the southern hemisphere.

Acting on this suggestion, subsequent astronomers have adopted the practice of considering the general average of all the stars, or a position which we may regard as their common center of gravity, to be at rest, and then determining the motion of the sun with respect to this center. Here we encounter the difficulty that we cannot make any absolute determination of the position of any such center. The latter will vary according to what particular stars we are able to include in our estimate. What we can do is to take all the stars which appear to have a proper motion, and determine the general direction of that motion. This gives us a certain point in the heavens toward which the solar system is traveling, and which is now called the solar apex, or the apex of the solar way.

The apparent motion of the stars due to this motion of the solar system is now called their parallactic motion, to distinguish it from the actual motion of the star itself.

The interest which attaches to the determination of the solar apex has led a great number of investigators to attempt it. Owing to the rather indefinite character of the material of investigation, the uncertainty of the proper motions, and the additions constantly made to the number of stars which are available for the purpose in view, different investigators have reached different results. Until quite recently, the general conclusionn was that the solar apex was situated somewhere in the constellation Hercules. But the general trend of recent research has been to place it in or near the adjoining constellation Lyra. This